1. Field of the Invention
This patent specification relates to electromagnetic measurements made in connection with boreholes. More particularly, this patent specification relates to methods and systems for correcting for, or determining attenuation due to, a conductive casing of a borehole used in making electromagnetic measurements.
2. Background of the Invention
Electromagnetic measurements are widely used within the oilfield industry to locate fluids inside the reservoir. Connate water is typically much more conductive than the hydrocarbon, and therefore measuring the resistivity of the formation allows clear distinction between the two fluids.
Conductivity measurements can be performed by inductive means. A number of induction tools have been developed for single well applications but other deployments have been proposed, such as cross-well EM, surface to borehole, borehole to surface and long offset single well, as a means to increase the depth of investigation around the borehole. A crosswell system is composed of a magnetic transmitter, which is located in one well, and some receivers located in a separate well. A surface to borehole system is composed of an array of transmitters at the surface and some receivers downhole, and a borehole to surface system is the opposite configuration of an array of transmitters downhole with receivers at the surface. A single well tool is composed of one or multiple transmitters located in the same well as one or multiple receivers.
Induction methods employing these systems do work well in open or fiberglass-cased wells, but when the well is steel cased, interpretation of the data becomes more problematic. This is due to the fact that the metal casing attenuates the signal and the casing imprint is not uniform as the casing properties (magnetic permeability, conductivity and thickness) are heterogeneous.
Some difficulties can be overcome when a non-magnetic steel casing is used (Chromium alloy for instance) but casing attenuation still needs to be accounted for in interpretation of the data.
A description of prior art regarding other techniques of removing the effects of conductive liners, or casing, in borehole EM can be found in U.S. Pat. No. 6,294,917 (hereinafter “the '917 Patent”), U.S. Pat. No. 6,393,363 (hereinafter “the '363 Patent,” and U.S. Patent Application Publication No. US2009/0005992A1 (hereinafter “the '992 Patent Application,” each of which is incorporated herein by reference. Some of the methods described rely on various ratios to cancel out the casing factor. Some of the methods rely on numerical modeling requiring a priori knowledge of the geologic structure surrounding the wells.
Thus it is desirable to provide methods for removing the effects of conductive liners that do not require a priori knowledge of the formation while not significantly degrading the sensitivity of the deep-sensing measurement to the formation near the wellbore.